The results of a new study suggest forensic scientists could one day use the microbial signature of people's privates to identify sexual offenders. Think of it as a musty, microscopic, x-rated fingerprint.

Historically, law enforcement has relied on hair samples to identify potential rapists, but pubic hairs recovered from crime scenes are often missing their roots, and therefore lack the genetic material to hold up as DNA evidence. Meanwhile, rapists are increasingly wearing condoms to avoid leaving genetic evidence in the form of semen.

Enter Murdoch University forensic biologist Silvana Tridico. Tridico and her team are investigating whether the microbial makeup of a person's pubic hair is unique enough to single someone out as a culprit. Here's Dalmeet Singh Chawla for Science Magazine:

Tridico and colleagues asked seven individuals—two of whom were living together—to collect their scalp and pubic hair for 5 months. The researchers then analyzed these samples in the lab, looking for bacterial populations present after 2 and 5 months. The scalp hair showed that 50 different varieties of microbes in males and 55 in females are found in this part of the body, but many of the microbes that were found were not specific to the individual carrying them. The pubic hair bacteria, however, turned out to be more distinct; in addition, each individual's "personal" pubic bacteria stayed roughly the same during the 5 months. More kinds of bacteria live in these hairs: approximately 73 in males and 76 in females. A larger combination of different bacteria means it is more likely for people to carry a unique microbial signature on them, Tridico says.

Although each person's pubic hair bacteria were distinct, the couple who were living together had greater similarity of bacteria on their pubic hairs at 5 months than detected after 2 months. The couple later revealed that they had sexual intercourse 18 hours before the collection of their hairs, the researchers report online today in Investigative Genetics.